In xerographic printing (also called electrophotographic printing), a latent image is formed on a charged photoreceptor, usually by raster sweeping a modulated laser beam across the photoreceptor. The latent image is then used to create a permanent image by transferring and fusing toner that was electrostatically attracted to the latent image onto a recording medium, usually plain paper.
While xerographic printing has been successful, problems arise when attempting to print at very high speed. One set of problems relates to the sweeping of the laser beam across the photoreceptor. As printing speed increases, it becomes more and more difficult to sweep the laser beam as fast as is required. While other sweeping methods are known, the most common method is to deflect the laser beam from a rotating mirror. Thus one way of increasing the sweep speed is to rotate the mirror faster. While this helps, extremely fast mirror rotation requires an expensive drive motor and bearings and an increasingly more powerful laser.
Other techniques of increasing the raster sweep speed are 1) to sweep the laser beam using a multifaceted, rotating polygon mirror (and a related set of optics), and/or 2) to sweep several laser beams simultaneously. Rotating polygon mirrors and related optics are so common that they are generically referred to as ROSs (Raster Output Scanners). Printers that sweep several beams simultaneously are referred to as multiple beam printers.
The raster sweep rate problem becomes even more apparent when printing in color at high speed. This is because a color xerographic printer requires a separate image for each color printed, hereinafter called a system color. While a dual color printer requires only two images, a full color printer typically requires four images, one for each of the three primary colors of cyan, magenta, yellow, and an additional one for black. Color prints are currently produced by sequentially transferring and fusing overlapped system colors onto a single recording medium which is passed multiple times, once for each system color, through the printer. Such printers are referred to as multiple pass printers. Conceptually, one can imprint multiple colors on a recording medium in one pass through the system by using a sequence of xerographic stations, one for each system color. If each station is associated with a separate photoreceptor, the printer is referred to as a multistation printer; if the stations use different positions on the same photoreceptor, the printer is referred to as a single station/multiposition printer. Multistation and single station/multiposition printers have greater printed page output than a multipass printer operating at the same raster sweep speed. However, the commercial introduction of multistation and single station/multiposition printers has been delayed by 1) cost problems, at least partially related to the cost of multiple xerographic elements and the associated ROSs, and 2) image quality problems, at least partially related to the difficulty of producing spots on each photoreceptor and then subsequently registering (overlapping) the images on the photoreceptor(s).
Proposed prior art multistation printers usually use individual ROSs (each comprised of separate polygon mirrors, lenses, and related optical components) for each station. For example, U.S. Pat. 4,847,642 to Murayama et al. involves such a system. Problems with such systems include the high cost of producing nearly identical multiple ROSs and the difficulty of registering the system colors.
A partial solution to the problems of multistation xerographic systems with individual ROSs is disclosed in U.S. Pat. No. 4,591,903 to Kawamura et al. That patent, particularly with regards to FIG. 6, discusses a recording apparatus (printer) having multiple recording stations and multiple lens systems, but only one rotating polygon mirror. Thus, the cost of the system is relatively low. However, differences in the lenses and mirror surfaces could still cause problems with accurate registration of different latent images.
Another approach to overcoming the problems of multistation printers having individual ROSs is disclosed in U.S. Pat. 4,962,312 to Matuura, et al. That patent illustrates spatially overlapping a plurality of beams using an optical beam combiner, deflecting the overlapped beams using a single polygon mirror, separating the deflected beams using an optical filter (and polarizers if more than two beams are used), and directing the separated beams onto associated photoreceptors. The advantage of overlapping the laser beams is a significant cost reduction since the ROS is shared. It is believed that a commercial embodiment of the apparatus disclosed in U.S. Pat. 4,962,312 would be rather complicated and expensive, especially if four system colors are to be printed. The use of optical beam combiners to overlap beams so that they have similar optical axes and similar sized spots is thought to be difficult, expensive, and time consuming.
One solution to the problems with the teachings of U.S. Pat. 4,962,312 is disclosed in "RASTER OUTPUT SCANNER FOR A MULTISTATION XEROGRAPHIC PRINTING SYSTEM," U.S. Pat. No 5,243,339issued Sept. 7, 1993 to Fisli. That patent provides a raster output scanning system employing a rotating polygon mirror that simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams having common optical axes and substantially common origins from common mirror surface areas. The clustered beams are subsequently separated by a plurality of optical filters and are then directed onto associated photoreceptors of a multistation printer. However, economically feasible optical filters require the dissimilar beams to be separated by a sufficiently large wavelength. Typically a wavelength difference of about 50 nm is required. For example, U.S. Pat. No. 5,243,359 utilizes lasers emitting at 645, 695, 755, and 825 nm. Since laser emission from closely spaced laser sources over this wavelength span is not yet available using one semiconductor material, practical systems need to integrate two distinctly different material systems, such as AlGaAs and AlGaInP. Additionally, the wide wavelength span necessitates that the photoreceptive surface(s) has adequate response over that span, which will include the infrared portions of the optical spectrum. However, few photoreceptive surfaces respond well in the infrared.
Accordingly, there is a need for apparatus and methods to simultaneously deflect and subsequently separate multiple, nearly coaxial laser beams emitted from closely spaced lasers having minimally different optical wavelengths. The apparatus and method should produce similarly dimensioned spots that are readily brought into registration.